1. Field of the Invention
This invention relates generally to replacements for vacuum tubes and in particular to a replacement for a vacuum tube that replicates the vacuum tube transfer characteristics including the small signal linear amplification and the large signal, i.e., overdrive, amplification.
2. Background of the Invention
Vacuum tubes once were widely used and widely available. However, as semiconductor technology has become more common place, vacuum tubes are now used in only a few applications. Today, one widespread application of vacuum tubes is in audio amplifiers that are used, for example, at concerts, in stereo systems, and in sound studios.
The human ear is a particularly sensitive gauge of the sound quality from an audio amplifier. Typically, important aspects in amplifying music are that the vacuum tubes, or any other devices used for audio amplification, faithfully replicate the input signals at low levels and that the vacuum tubes slowly and uniformly compress the input signals at higher drive levels and under overdrive conditions. Vacuum tubes have these characteristics and it is these characteristics that result in amplified music that is pleasing to the human ear.
FIG. 1 is a set of characteristic curves 100 for a triode vacuum tube. Horizonal axis 110 is the plate to cathode voltage while vertical axis 120 is the plate current. Each curve 101 to 109 corresponds to a different grid voltage, and there is a one volt change from curve to curve. The curves were generated with a 65 Kohm load. As is known to those skilled in the art, it is important to have the complete set of characteristics curves, i.e., both the positive and negative grid voltages. Any device, that replicates only the negative grid voltage curves and is used in an audio amplifier, fails to slowly and uniformly compress the input signals at higher drive levels and under overdrive conditions.
While vacuum tubes have the desired characteristics for audio amplifiers, the vacuum tube industry is practically non-existent in the United States. Most vacuum tubes are currently being manufactured in foreign countries and unfortunately, not only is the availability of vacuum tubes limited, but also the quality of the vacuum tubes is highly variable. Further, as discussed below, there is not a suitable semiconductor replacement for vacuum tubes.
Bipolar transistors have been used in audio amplifier applications. However, bipolar transistors exhibit non-uniform saturation conditions; non-uniform transfer for amplification which generates odd order harmonics and high output impedance which produce unpleasing sound and so are not suitable for replicating the overdrive characteristics of a vacuum tube.
A short channel JFET 200 has non-saturated current voltage (I-V) characteristics which are similar to those of a vacuum tube triode. The drain of short channel junction field effect transistor (JFET) 200 is a heavily doped region 201 of conductivity type N++. Drain 201 is overlain by a lightly doped epitaxial layer 202 of conductivity type N-. As is known to those skilled in the art, a portion of region 202 functions as the channel of JFET 200.
Two pockets 203, 204 of conductivity type P+, with their centers separated by a distance "a" (the device pitch), extend into epitaxial region 202. Approximately centered between pockets 203, 204 in epitaxial region 202 is a doped source region 205 of conductivity type N+ or N++. An insulating layer 206 (in FIG. 2, a letter is used after reference numeral 206 to identify the different regions of the insulating layer that are visible in the cross-sectional view) overlies surface 202A and has contact openings over pockets 203, 204 and a contact opening over source 205.
Metal gate electrodes 210, 212 electrically contact pockets 203 and 204 respectively and overlie insulating layer 206 so that portions of electrodes 210 and 212 which are not in electrical contact with pockets 203 and 204 are electrically insulated from region 202. Source electrode 211 electrically contacts source 205 and also overlies insulating layer 206 so that portions of electrodes 211 which are not in electrical contact with source 205 are electrically insulated from region 202. A passivation layer 213 overlies electrodes 210, 211, 212 and insulating layer 206.
FIG. 3 is a graph of the I-V characteristics for short channel JFET 200. The ordinate of the graph is the drain-to-source current in amps (I.sub.DS [A]) the abscissa is the drain-to-source voltage in volts (V.sub.DS [V]). Curves 301, 302, 303, 304, and 305 are for gate-to-source voltages (V.sub.GS [V]) of 0, -2, -4, -6, and -8 volts respectively.
The drain current of JFET 200 (FIG. 2) is controlled by the (negative) gate potential, as well as the (positive) drain potential. The drain current decreases as the magnitude of the (negative) gate voltage increases. Moreover, the drain current increases with a rise in the drain voltage, a "short channel"-like behavior. Conditions under which semiconductor devices exhibit triode-like I-V characteristics are well-known to those skilled-in-the-art. For example see, G. F. Neumark, E. S. Rittner, "Transition from pentode- to triode-like characteristics in FETs", SSE, V10, pp. 299-304, 1967. Other references that define the prior art include: W. Shockley, "Transistorelectronics: imperfections, unipolar and analog transistors", PIRE, V40, pp. 1289-1313, November 1952; R. Zuleeg, "Multi-channel FET theory and experiment", SSE, V10, pp. 559-576, 1967; Chiu & Ghosh, "Characteristics of Junction-Gate Field Effect Transistor with Short Channel Length", Solid State Electronics, Vol. 14, pp. 1307-1317, 1971. C. Kim, E. Yang, "Carrier accumulation and space-charge-limited current flow in FETs", SSE, V13, pp. 1577-1589, 1970; J. Nishizawa, T. Terasaki, J. Shibata, "FET versus Analog Transistor (Static Induction Transistor)" IEEE Transaction on Electron Devices, Vol. ED-22, No. 4, April 1975; A. S. Wang, C. J. Dell'Oca, "A compatible bipolar and JFET Process" IEDM Proc., pp. 45-47, December 1976; J. Nishizawa, Semiconductor Technology in Japan, Chapter 15, North Holland, Publisher, New York, 1982; M. G. Kane, R. Frey, "The PSIFET emerges as a new contender", MSN, pp. 46-58, September 1984; A. Cogan et al. "Progress toward the ultimate semiconductor switch", Powertechniques Magazine, pp. 35-39, September 1986; J. Browne, "Solid State Triodes boost high voltages at broad bandwidths", Microwaves & RF, pp. 221-224, May, 1989; B. J. Baliga, "Bipolar operation of power JFETs.", EI.Letters, V10, No. 2, February 1980.
While prior art short channel JFET 200 provides enhanced performance in audio applications over equivalent bipolar and MOS transistors, short channel JFET 200 does not have the overdrive characteristics of a triode vacuum tube. When the gate-to-source junction of JFET 200 (FIG. 2) becomes forward biased, e.g., when the source input signal increases above a certain threshold or power level, i.e, JFET 200 is overdriven, the junction injects minority carriers into the drain-to-source channel. Consequently, unipolar JFET 200 becomes a bipolar transistor, i.e, the input signal has exceeded the bipolar turn-on threshold and JFET 200 may be destroyed by excess current. Also, as described above, a bipolar transistor has does not have the necessary characteristics to replicate the overdrive characteristics of a vacuum tube. Thus, while it is generally known that a short channel JFET 200 has nonsaturated current voltage (I-V) characteristics which are similar to those of a vacuum tube triode, short channel JFET 200 cannot provide the overdrive characteristics that are an important part of music amplification. Consequently, to the best of the inventors knowledge, there is not a suitable replacement for a vacuum tube used in an audio amplifier.